Ballasts which use electronic switches such as triacs in phase control circuits for achieving close regulation together with controlled variation of discharge lamp current are known. Willis U.S. Pat. No. 3,873,910, Ballast Control Device, discloses such a ballast comprising an inductor having two windings separated by a shunt providing magnetic leakage. One winding, conveniently termed the main or running winding, is connected in series with the lamp across the A.C. line. The other winding is a control winding and has an electronic switch such as a triac connected across it. The inductance of the main winding when the switch in the control winding is open is determined by the turns in the main winding and the total magnetic path of the core structure. When the switch is closed, the current induced in the control winding produces a flux opposing the main winding flux. As a result, the main flux is forced through the shunt and its included air gap which make a magnetic path of higher total reluctance. This lowers the effective inductance of the main winding and permits the current to increase, thus increasing lamp wattage. By appropriately varying the firing phase angle, that is the moment in the A.C. cycle at which the triac is fired, the wattage input into the lamp may be regulated to achieve constant light output notwithstanding line voltage variations.
The firing phase angle of the triac may also be varied to control light output, that is to dim or brighten the lamp. However, when the control winding is shorted by a phase controlled triac the current goes to a higher level as the inductance is lowered. If the difference in lamp current between open control winding and shorted control winding is too great, the lamp voltage rises as a result of current starvation at the beginning of each half cycle. Thus this method of control causes line current distortion in the form of high third harmonic and high instantaneous lamp voltage which can cause premature lamp dropout. These problems limit the practical range of wattage control, although the control range is entirely sufficient for maintaining lamp wattage constant as against line voltage variations.
In order to economize energy, more and more industrial and commercial lighting installations now include means for varying the light level depending upon time of day and the activity going on. In one such arrangement known as remote energy management, the light levels at various places are controlled by signals from a central location. Typically, it is desired to control the input wattage into a high pressure sodium vapor lamp in 50-watt steps starting at 150 watts and going up to 450 watts. In order to use the Willis circuit, the initial approach was to have two running windings together with one control winding. One running winding, known as the main winding, was used for the high wattage range (300-450 watts) which requires low reactance. Both running windings, the main and an extended winding, were connected in series for the low wattage range (150-250 watts) which requires higher reactance. Switching from the high range to the low range was done by a relay.
In the foregoing arrangement the limits of each control range must be adequate to accommodate the upper wattage setting at low line voltage and the lower wattage setting at high line voltage. The lower wattage limits are determined by the number of turns on the two running windings taken together and the main magnetic path reluctance with the control winding open. The main magnetic path generally includes an air gap which contributes the largest part of the reluctance. The upper wattage limits are determined by the magnetic shunt path with the control winding shorted. When the limits are set as tight as possible for the low wattage range, the high wattage range limits are much too wide. This happens because when fewer main winding turns are selected for the high wattage range, the effective flux leakage between main winding and control winding is reduced so that the control winding has more effect on the reactance.